Output feedback and under-voltage detection system that senses an input current representing a voltage input

ABSTRACT

A switched mode controller for properly handling an under-voltage condition in a power line which includes a current mirror for receiving current from the power line; a reference current source coupled to the current mirror for supplying a reference current; and a power transistor coupled to the reference current source, the power transistor generating a pulse width modulated signal when current from the power line exceeds the reference current, the power transistor being disabled when current from the power line is less than the reference current.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Divisional of U.S. application Ser. No. 09/192,960, filed Nov.16, 1998, now U.S. Pat. No. 6,147,883.

BACKGROUND

The present invention relates to a switched mode control system withoutput feedback and under-voltage detection.

Quantum leaps in electronic technology have led to the development of“smart” electrical and electronic products. Each of these productsrequires a steady and clean source of power from a power supply. In onecommon power supply known as a switching power supply, an integratedcircuit (IC) regulator is connected in series with a primary winding ofa transformer to the high voltage direct current (DC) input derived fromalternating current (AC) power line through rectification and filtering.Energy is transferred to the secondary winding in a manner controlled bythe IC regulator so as to provide a clean and constant output voltage.Another winding called a feedback or bias winding may be used to providea feedback signal to the IC regulator. Alternately, the feedback signalcan come through an opto-coupler from a sense circuit at the secondaryoutput. The feedback signal is used to modulate the duty cycle of the ICregulator or used to allow or disallow cycles in order to control thesecondary output.

When the power supply is turned on or off, a temporary uncertaintyexists in the output of the power supply. The uncertainty leads totransients or glitches in the output voltage. The condition may causethe output of the power supply to fluctuate unpredictably. For instance,the power supply output may go to zero volts and come back up partiallyseveral times during power-down (glitch). Power supplies withauto-restart circuit, which are designed to restart the power supplyperiodically under fault conditions, are susceptible to such glitchesduring power-down. Since digital and analog ICs used in modern productsare highly sensitive to fluctuations in their voltage supply, such anunder-voltage condition may lead to erratic or inoperative products.

SUMMARY

A circuit detects an under-voltage condition at a voltage input of apower supply. In one aspect, the circuit has a current mirror forreceiving current representing the voltage input to the power supply;and a reference current source coupled to the current mirror to generatean under-voltage signal when the reference current exceeds the currentfrom the power supply voltage input. The power supply is disabled whenthe under-voltage signal is generated.

Implementations of the invention include one or more of the following.The under-voltage condition is detected during power up and/or during anoff cycle of an auto-restart operation of the power supply. A resistormay be connected to the power supply input voltage and a first isolationtransistor may be connected between the resistor and the current mirror.The current mirror may include a first transistor having a gate and adrain, the gate being connected to the drain and a second transistorhaving a gate, the gate of the second transistor connected to the gateof the first transistor. The reference current source may be connectedto the second transistor. The resistor may be connected to the powersupply voltage input at an electrical terminal. A second current sourceand a second isolation transistor may be connected between the secondcurrent source and the electrical terminal to receive a feedback signal.A latch may be connected to the output of the second reference currentsource to receive the feedback signal. The electrical terminal receivesboth the feedback signal and the under-voltage signal. Further, only oneof the first and second isolation transistors may conduct at a time.

In a second aspect, a method for handling an under-voltage condition ata voltage input of a power supply includes sensing an input currentrepresenting the voltage input of the power supply; generating areference current; comparing the input current with the referencecurrent; detecting the under-voltage condition when the referencecurrent exceeds the input current; and disabling the power supply whenthe under-voltage condition is detected.

Implementations of the invention include one or more of the following.The under-voltage condition may be detected during power up and duringan off cycle in an auto-restart operation of the power supply. The inputcurrent may be mirrored by passing the input current through a firsttransistor having a gate and a drain, the gate being connected to thedrain; and passing the input current through a second transistor havinga gate, the gate of the second transistor connected to the gate of thefirst transistor. The input current and a feedback signal may bereceived on one electrical terminal. A second reference current sourcemay be connected to the electrical terminal to receive a feedbacksignal. Current flow from the second current source to the electricalterminal may be isolated when detecting the under-voltage condition. Thefeedback signal from the electrical terminal may be latched. The methodmay include isolating the electrical terminal and a second currentsource during predetermined periods. Where a second isolation transistoris connected between the electrical terminal and the current mirror, themethod may include comprising conducting each of the first and secondisolation transistors one at a time. The method also includes detectingif a fault condition exists and if so, disabling a power switchingtransistor in the power supply.

In another aspect, a regulator circuit is connected in series with atransformer primary winding to an input voltage. The regulator circuitincludes a resistor coupled to the input voltage, the resistor providingan input current representing the input voltage; a current mirrorcoupled to the resistor for mirroring the input current; and a referencecurrent source coupled to the current mirror to generate anunder-voltage signal when the reference current exceeds the inputcurrent, the power supply being disabled when the under-voltage signalis generated.

Implementations of the invention include one or more of the following.The under-voltage signal is detected during power up and/or during anoff cycle in an auto-restart operation of the regulator circuit. Atransistor may be connected between the resistor and the current mirror.The current mirror may include a first transistor having a gate and adrain, the gate being connected to the drain and a second transistorhaving a gate, the gate of the second transistor connected to the gateof the first transistor. A second reference current source may be used,and a first isolation transistor may be connected between the secondcurrent source and the resistor. A latch may be connected to the outputof the second reference current source for receiving a feedback signal.The feedback signal and the under-voltage signal may be received by oneelectrical terminal. A second isolation transistor may be connectedbetween the electrical terminal and the current mirror, wherein each ofthe first and second isolation transistors conducts one at a time. Apower switching transistor may be connected to the under-voltage signal.

In another aspect, a power supply circuit has an under-voltage detectorfor receiving a signal representing an input voltage on an electricalterminal to detect under-voltage condition, the power supply beingdisabled when the under-voltage condition is detected. The circuit has acircuit for receiving a feedback signal on the electrical terminal. Thefeedback signal is used to regulate the power supply output.

Implementations of the invention include one or more of the following.The under-voltage signal may be a current proportional to the inputvoltage. The current may flow through a resistor coupled between theinput voltage and the electrical terminal. The feedback signal may bedigital.

In yet another aspect, a power supply includes an auto-restart circuitfor alternatingly disabling and enabling the power supply when the powersupply is under a fault condition. The power supply also has an inputunder-voltage detector to detect an input under-voltage condition whenthe power supply is disabled by the auto-restart circuit and duringpower-up.

An implementation of the invention may prevent the power supply frombeing enabled following the detection of the input under-voltagecondition until the input under-voltage condition is removed.

In another aspect, a method for handling an under-voltage condition atan input voltage of power supply circuit includes receiving an inputsignal representing the input voltage on an electrical terminal, theelectrical terminal carrying the input signal and a feedback signal toregulate the power supply output; providing feedback control for thepower supply based on the feedback signal carried by the electricalterminal; detecting an under-voltage condition based on the inputsignal; and disabling the power supply when the under-voltage conditionis detected.

In another aspect, a method for handling an under-voltage condition atan input voltage of power supply circuit includes detecting an inputunder-voltage condition during the disabling of the power supply andduring power-up; and alternatingly disabling and enabling the powersupply when the input under-voltage condition is present.

Advantages of the invention include one or more of the following. Thepower supply output behaves in a predictable manner when power is turnedon and off or during input power interruptions. The invention preventsglitches at the output when power is turned on and off. The inventionrequires only one resistor external to the regulator to sense inputvoltage. The number of external components is reduced only one input isrequired to supply feedback information and under-voltage information tothe regulator. Thus the number of electrical terminals required for theregulator is reduced. The reduction in electrical terminals in turnreduces circuit area in the regulator dedicated to protect the regulatoragainst latch-up and electrostatic discharge (ESD) conditions. Further,a small package with a low pin count may be used. Such a small packagereduces printed circuit board (PCB) wiring complexities and PCB area.The reduction in terminal count also allows one or more terminals to beused as an extra thermal conduction path between the regulator to a heatsink on a PCB for rapid heat dissipation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a circuit for receiving output feedbackand detecting under-voltage conditions on the AC line.

FIG. 2 is a schematic of a power supply.

FIG. 3 is a timing diagram illustrating the operation of the presentinvention.

DESCRIPTION

Referring now to FIG. 1, a schematic diagram of a circuit for receivingfeedback from the output and for detecting an under-voltage condition inthe power line is shown. The circuit may be provided within an ICregulator. A first reference current source 102 and a second referencecurrent source 104 in the regulator are connected to an external by-passcapacitor at the supply terminal (by-pass) of the regulator. The currentreference sources 102 and 104 may be p-channel reference current sourcesand each reference current source may provide about 50 micro-amperes ofcurrent.

The output of the current source 102 is provided to a node 109 which isconnected to the drain of a transistor 106. Transistor 106 may be ann-MOS transistor whose gate is tied to the sum of 1.5 volts and athreshold voltage. The source of transistor 106 is connected to afeedback/under-voltage terminal 107. Terminal 107 is connected to aresistor 132 (FIG. 2) which is connected to the input DC voltage of thepower supply. Current flowing through resistor 132 (FIG. 2) is used todetect under-voltage conditions.

Also connected to the feedback/under-voltage terminal 107 is transistor108 which may be a p-MOS transistor. The p-MOS transistor 108, whosegate is tied to 1.5 volts, isolates an under-voltage detector (discussedbelow) from the rest of the circuit. Transistor 108 in turn is connectedto a transistor 110 whose gate is commonly tied to the gate of anothertransistor 112. The gate and drain of transistor 110 are connected. Bothtransistors 110 and 112 may be n-MOS type transistors. The combinationof transistors 110 and 112 form a current mirror. The output of thetransistor 112 is connected to the reference current source 104 at ajunction 113 which provides a “LINE-UNDERVOLTAGE” signal to a faultdetection circuit 116. In this embodiment, transistors 108, 110 and 112,along with reference current source 104, form the under-voltagedetector.

Junction 109 between the reference current source 102 and transistor 106drives one input of an AND-gate 118. The other input of the AND-gate 118is connected to an oscillator 114 which supplies a clock signal for theregulator. The output of the AND-gate 118 is provided to a set input ofa latch 122. A second output from the oscillator 114 which supplies asecond clock signal with a maximum clock duty cycle is provided to aninverted input of an OR-gate 120. The other input of OR-gate 120receives a current limit signal from the regulator. The output ofOR-gate 120 is provided to a reset input of the latch 122.

The output of the latch 122 is provided to one input of an AND-gate 124.Another input of AND-gate 124 is provided by auto-restart circuit 116which performs fault detection. The third input of AND-gate 124 isprovided by the bypass pin regulator and power-up circuit 117. Thisinput will be in the low state during power-up until the bypass pinreaches its regulated voltage of 5.8 volts.

The auto-restart circuit 116 receives an input under-voltage signal fromjunction 113 between the transistor 112 and the reference current source104. If transistor 112 pulls more current than current source 104 cansupply, junction 113 goes low which indicates that an inputunder-voltage condition does not exist. Alternatively, if transistor 112pulls less current that supplied by the current source 104, junction 113goes high, indicating that an input under-voltage condition does exist.The auto-restart circuit alternatively enables and disables the powertransistor 126 when the power supply is experiencing fault conditionssuch as output overload or output short. The presence of an inputunder-voltage condition during the off cycle (disable) of auto-restartcauses circuit 116 to continue asserting a fault signal to AND-gate 124until the input under-voltage condition goes away. The bypass pinregulator and power-up circuit 117 regulates the bypass pin to 5.8volts. There is an external capacitor connected to this pin. Duringpower-up the bypass pin capacitor is charged by the bypass pin regulatoruntil it reaches its regulated voltage of 5.8 volts. However, duringpower-up if the input under-voltage condition exists, the bypass pinwill charge up only to 5.1 volts and held there. When the inputunder-voltage condition goes away, the bypass pin capacitor will chargeup from 5.1 volts to its regulated voltage of 5.8 volts. The output ofAND-gate 124 in turn is provided to the gate of power transistor 126connected between a drain input and a source input of the regulator. Ifa fault signal is asserted or if bypass pin has not reached 5.8 voltsduring power-up, AND-gate 124 shuts off the power transistor 126. Inthis manner, the input under-voltage condition causes power transistor126 to be disabled until the input under-voltage condition goes away.

The circuit of FIG. 1 also receives a feedback signal throughfeedback/under-voltage terminal 107. During operation, when current ispulled out of terminal 107, the voltage at terminal 107 is lower thanthe voltage at the gate of the transistor 108 (for example, 1.5 volts),causing transistor 108 to turn off and isolate the under-voltagedetection circuitry. Further, transistor 106 is turned on, allowingcurrent to flow through the transistor of optocoupler 144 (FIG. 2). Ifthe current flowing through the transistor of optocoupler 144 atterminal 107 is less than the current supplied by the reference currentsource 102 at node 109, node 109 goes high. When node 109 is high,AND-gate 118 passes the clock signal from oscillator 114 to the setinput of latch 122. Such setting of latch 122 in turn enables powertransistor 126 to provide a pulse modulated signal to the transformer inthe absence of a fault condition. Alternatively, if the current flowingthrough the transistor of optocoupler 144 at terminal 107 is more thanthe current supplied by the reference current source 102 at node 109,node 109 goes low, thus disabling AND-gate 118 and power transistor 126.Although the feedback signal in this case is analog in form, it can alsobe a digital signal.

In one embodiment of a power supply, the current through the transistorof optocoupler 144 is digital in nature, where when the output voltageis below its regulated voltage there is no current through optocoupler144, and when the output voltage is above the regulated voltage there issufficient current through optocoupler 144 to pull terminal 107 low.

FIG. 2 illustrates a power supply employing the feedback andunder-voltage handling circuitry of FIG. 1. The power supply of FIG. 2has a transformer 134 with a primary winding and a secondary winding. Afirst input of the primary winding of the transformer 134 is connectedto the positive input voltage terminal, while a second input of theprimary winding of the transformer 134 is provided to a drain input D ofa regulator 146 which embodies the circuit of FIG. 1. Additionally, thesecond input of the primary winding of the transformer 134 is connectedto a clamp circuit consisting of a resistor 150 and a capacitor 152connected in series to the input negative terminal. A source terminal Sof the regulator 146 is also connected to the input negative terminal.Further, a by-pass terminal BP of regulator 146 is connected to one endof capacitor 148. The other end of the capacitor 148 is connected to theinput negative terminal. Capacitor 148 gets charged by the by-passterminal BP of regulator 146 during power up, and after the by-passterminal reaches 5.8 volts, the power transistor 126 can startswitching. Also a resistor 132 is connected between the input voltageterminal and a feedback/under-voltage terminal of regulator 146.

One output of the secondary winding of transformer 134 is provided to aSchottky diode 136 which is connected to a capacitor 138. The otheroutput of the secondary winding of the transformer 134 is connected tothe output negative terminal. The output of diode 136 is connected to asecond Zener diode 140 at one end. The other end of Zener diode 140 isconnected to a resistor 142, the other terminal of which is connected tothe output negative terminal. The other end of the Zener diode 140 isconnected to an optoisolator 144 whose output is connected between thefeedback/under-voltage terminal of the regulator 146 and the outputnegative terminal.

FIG. 3 is a timing diagram illustrating the operation of the circuits ofFIGS. 1 and 2. Referring to all three figures in period 150, inputvoltage V_(IN) rises from zero volts during power up. The input voltagerises from zero past a predetermined threshold to reach its fullvoltage. In this example, the threshold is set to be at about 100 volts.When the input voltage passes 100 volts, the drain of power transistor126 begins to switch and to regulate the power output. Alternatively,when the input voltage is below 100 volts, the by-pass terminal of theregulator 146 is prevented from charging to its full voltage of about5.8 volts. This is accomplished by holding the by-pass terminal voltageat about 5.1 volts using an internal voltage regulator (not shown) toprevent power transistor 126 from switching.

Period 152 begins after the input voltage has exceeded the threshold of100 volts. In this period, the by-pass terminal voltage is allowed torise to about 5.8 volts to complete the power up sequence. After theby-pass terminal voltage passes about 5.8 volts, power transistor 126switches at its frequency of approximately 130 KHz in this example.Since the output voltage starts at zero volts, it will take apredetermined time period (typically two to ten milliseconds) for outputcapacitor 138 to charge up. Thus, at the beginning of period 152, nocycle is skipped. Once capacitor 138 reaches its full output voltage, itthen enters a regulation mode and cycles may be skipped, as illustratednear the end of period 152. Here, the turn-on time associated with theauto restart circuit is longer than the normal power up time required tocharge the output capacitor 138. Towards the end of the period 152,regulator 146 is regulating properly.

Auto-restart operation of the power supply is discussed next.Auto-restart operation is defined as alternatively enabling anddisabling the power supply when it is experiencing one or more faultconditions such as over-load, open-loop, and short-circuit conditions.The disabling period is normally longer than the enabling period tolimit excessive power delivery. The alternating enabling and disablingperiods continue until the fault condition is removed.

Period 154 illustrates the auto-restart operation of regulator 146 whena short condition exists on the output of the power supply of FIG. 2(D.C. OUT). Once the output is shorted, the output voltage collapsesfrom its nominal voltage (in this case, about five volts) to ground.This condition demands that the regulator does not skip any switchingcycles over a predetermined duration, typically 32 milliseconds. Whenthe demand for full cycling exists for more than 32 milliseconds,regulator 146 detects a fault condition on the power supply output (D.C.OUT) and initiates the auto-restart operation where the power transistor126 is prevented from switching for another period such as 128milliseconds. The disabling of power transistor 126 during this periodavoids the possibility of too much energy being shunted to output diode136. After the expiration of the 128 milliseconds, regulator 146 retriesstarting the power supply for 32 milliseconds to see if the shortcondition has disappeared.

Before trying a restart, regulator 146 ensures that the input voltage isnot low. Two possible situations may have created a continuous demandfor cycling energy beyond the duration of 32 milliseconds: 1) a shortcondition or 2) an under-voltage condition. By checking for an inputunder-voltage condition before starting the power supply through theauto-restart circuit, the fault condition may be properly handled.

In period 156, the short condition is removed and the output isregulated as normal. In period 158, when power is removed, the linevoltage starts to drop. Even as the line voltage drops, enough powerexists to allow regulator 146 to provide a regulated output duringperiod 158. Eventually, in period 160, the input voltage falls to alevel where the output of the power supply goes out of regulation andthe feedback signal demands the regulator 146 to provide maximum power(no skipped cycles) for the period of 32 milliseconds. Meanwhile, sincethe input voltage is low in period 160, the output starts to decreaseand less energy is provided to the secondary winding of transformer 134.At the end of period 160, regulator 146 initiates the auto-restartoperation and prevents transistor 126 from switching. In period 162,after the 128 millisecond period, regulator 146 checks the input voltageprior to restarting. If the input voltage is below the threshold, powertransistor 126 is disabled to prevent the restart attempt to eliminatepower-down glitches at the output. Such power-down glitches may resultin equipment malfunction, as the output voltage may drop from 5 volts tozero volts, then 128 milliseconds later, the regulator 146 may retry andcause the output to rise from zero to approximately 2.5 volts, forexample, before collapsing to ground. This could repeat several timesdepending on the rate of decay of input voltage.

As shown in this example, input voltage is checked during the power-upor during the off cycle of auto-restart operation. Detectingunder-voltage prevents power transistor 126 from starting to switch.This operation eliminates power-up and power-down glitches that wouldotherwise occur as a result of auto-restart.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the size,shape, materials, components, circuit elements, wiring connections andcontacts, as well as in the details of the illustrated circuitry andconstruction and method of operation may be made without departing fromthe spirit of the invention.

What is claimed is:
 1. A method for handling an under-voltage conditionat a voltage input of a power supply comprising: sensing an inputcurrent representing the voltage input of the power supply; generating areference current; detecting the under-voltage condition when thereference current exceeds the input current; and disabling the powersupply when the under-voltage condition is detected.
 2. The method ofclaim 1 further comprising detecting the under-voltage condition duringpower up and/or during an off cycle in an auto-restart operation of thepower supply.
 3. The method of claim 1 further comprising mirroring theinput current, including: passing a mirrored input current through afirst transistor having a gate and a drain, the gate being coupled tothe drain; and passing the mirrored input current through a secondtransistor having a gate, the gate of the second transistor coupled tothe gate of the first transistor.
 4. The method of claim 1 furthercomprising receiving the input current and a feedback signal on anelectrical terminal.
 5. The method of claim 3 further comprisingproviding a second reference current source coupled to the electricalterminal to receive a feedback signal.
 6. The method of claim 5 furthercomprising isolating current flow from the second current source to theelectrical terminal when detecting the under-voltage condition.
 7. Themethod of claim 5 further comprising passing the feedback signal fromthe electrical terminal to a latch.
 8. The method of claim 5 wherein afirst isolation transistor is coupled between the electrical terminaland the second current source, further comprising isolating theelectrical terminal and the second current source during predeterminedperiods.
 9. The method of claim 8 wherein a second isolation transistoris coupled between the electrical terminal and the current mirror,further comprising conducting each of the first and second isolationtransistors one at a time.
 10. The method of claim 1 further comprisingdetecting if a fault condition exists and if so, disabling a powerswitching transistor in the power supply.
 11. A power supply circuithaving an input voltage comprising: an under-voltage detector to receivean input signal representing the input voltage on an electrical terminalto detect an under-voltage condition, wherein the input signal is acurrent proportional to the input voltage, the power supply beingdisabled when the under-voltage condition is detected; and a circuitcoupled to the electrical terminal for receiving a feedback signal toregulate the power supply output.
 12. The power supply circuit of claim11 wherein the current flows through a resistor coupled between theinput voltage and the electrical terminal.
 13. The power supply circuitof claim 11 wherein the feedback signal is digital.
 14. A method forhandling an under-voltage condition at an input voltage of power supplycircuit comprising: receiving an input signal representing the inputvoltage on an electrical terminal, wherein the input signal is a currentproportional to the input voltage, the electrical terminal carrying theinput signal and a feedback signal to regulate the power supply output;providing feedback control for the power supply based on the feedbacksignal carried by the electrical terminal; detecting an under-voltagecondition based on the input signal; and disabling the power supply whenthe under-voltage condition is detected.
 15. The method of claim 14wherein the current flows through a resistor coupled between the inputvoltage and the electrical terminal.
 16. The method of claim 14 whereinthe feedback signal is digital.
 17. A method for handling anunder-voltage condition at an input voltage of a power supply circuitcomprising detecting an input under-voltage condition when the powersupply is disabled in auto-restart and during power-up, whereindetecting the input-under voltage condition includes sensing an inputcurrent representing the input voltage of the power supply, generating areference current and detecting the under-voltage condition when thereference current exceeds the input current.
 18. The method of claim 17further comprising preventing the power supply from being enabledfollowing the detection of the input under-voltage condition until theinput under-voltage condition is removed.